Fixed-depth is worse than pointless. It favors the engine that prunes least, i.e. that wastes most time on searching poor lines. Even TSCP would beat Houdini at 16 ply. (It would take a few months per move to reach 16 ply, of course.)

Standard is fixed time per game or session, where engines that won't waste time on poor lines, but use that time to reach higher depth on the promising lines instead, are strongest.